1. Field of the Invention
This invention relates to systems and methods for moving semiconductor wafers in a semiconductor manufacturing equipment.
2. Description of the Related Art
Specialized semiconductor manufacturing equipment is used to convert bare silicon wafers into working devices. This semiconductor manufacturing equipment typically has a loading station for placing pre-processed wafers therein, a process module for processing the wafers, and a wafer handling system for moving wafers between the loading station and the process module.
In modern semiconductor manufacturing equipment, the wafer handling system incorporates automated mechanical manipulators or robots. The robot has a paddle or end-effector at the end of its arm for picking up a wafer and placing the wafer at various locations within the semiconductor manufacturing equipment. The robot can automatically perform its wafer handling tasks because the coordinates of various locations within the manufacturing equipment, such as process modules and load locks, are pre-programmed in the robot. To move a wafer from one location to another, for example, the robot can determine its present position by reading various sensors and encoders, recall from its storage memory the pre-programmed coordinates of the destination location, and then move the wafer along a calculated path onto the destination location. There are several factors that affect the robot""s ability to accurately move wafers. First, it is critical that the placement/pick-up locations within the manufacturing equipment do not change after the robot is programmed with their coordinates. This is usually not a problem because the modules comprising the manufacturing equipment are fixedly installed onto the factory floor. As long as the equipment""s modules are not replaced or moved, the programmed coordinates should remain valid indefinitely. Second, wafer movement within the manufacturing equipment is based on the assumption that the wafer is centered on the robot""s paddle as the wafer is moved from one location to another. Unlike the pre-programmed location coordinates, the position of the wafer on the paddle cannot be pre-determined nor assumed. The position of the wafer on the paddle can be shifted due to hand-off variations between paddle pre-programmed coordinates or due to movement in the process module, load lock, or loading station. Off-center wafers can result in wafer breakage and/or wafer misplacement. Thus, it is very important to accurately determine how much the wafer is off-centered such that necessary corrections can be made prior to placing the wafer onto a location.
The invention provides a system and method for detecting the position of a wafer with respect to a calibrated reference position. In one embodiment of the invention, sensors are used to detect the edges of the wafer as the wafer is passed over the sensors. This edge detection information is then used to calculate the amount by which the wafer deviated from a reference position so that the robot""s placement coordinates can be modified to compensate for the deviation before the wafer is placed onto a destination location.